1. Field of the Invention
The present invention relates to an image forming apparatus configured to discharge a sheet on which an image is formed to a space between a main body of the image forming apparatus and an image reading device and, in particular, to a structure to prevent a disorder in a stack of discharged sheets.
2. Description of the Related Art
Conventionally, as an exemplary image forming apparatus, other than facsimile machines or printers, there exists an image forming apparatus in which an image reading device is additionally arranged on a main body of the image forming apparatus in order to add another function such as a copying function. In a case of such an image reading device, since the apparatus grows in size, a space is provided between the image reading device and the main body of the image forming apparatus to discharge sheets thereinto, thereby achieving space saving.
Herein, the image forming apparatus of an internally-sheet-discharging type with such a structure, at a time of image formation, feeds a sheet to an image forming portion to form a toner image on the sheet and thereafter, fixes the toner image on the sheet by means of a fixing portion. Next, the sheet on which the toner image is fixed is discharged and stacked on a sheet discharge tray provided in a space between the image reading device and the image forming apparatus main body.
Herein, the sheet heated up by the fixing portion sometimes becomes greatly curled, i.e., has a curl depending on a condition. If the sheet in such a curled condition is stacked on the sheet discharge tray, the sheet may clog a discharge outlet and in this case, the stacked sheet is pushed out by the next discharged sheet, thereby causing damage to alignment or dropping the sheet from the discharge tray.
It is to be noted that a curl size or direction of the sheet depends on a type of sheet, a basis weight (unit weight), a fiber orientation, an environmental temperature, the amount of toner in a toner image for forming an image on the sheet, a fixing temperature, and the like. As such a curl, there exists a U-shaped curl that both ends parallel to a sheet conveying direction of the sheet are turned up.
A demand for an image forming apparatus of a desktop type which is placed on a desk during use has been increased in recent years. Herein, since such a desktop type image forming apparatus needs to reduce its height, it is necessary to reduce a height in a vertical direction of a discharge space between the discharge tray and the image reading device, in addition to a height of the image forming apparatus main body and a thickness of the image reading device itself. However, as the discharge space is reduced in height, the sheet to be discharged is discharged while being in contact with the bottom surface of the image reading device. Therefore, in order to secure a discharge conveyance ability of sheet, such a conventional structure has been proposed that the discharge tray and the bottom surface of the image reading device are faced to each other (see, U.S. Patent Application Publication No. 2005/281598 A1).
With such a conventional image forming apparatus, when a sheet is curled and a discharge space is low in height, the number of stacked sheets decreases. Especially where a sheet is curled in a U-shape, because of strength of the sheet, sheets S are linearly stacked on the discharge tray 150 one by one along an inclination of the discharge tray 150, as illustrated in FIG. 7.
When the sheets S are linearly stacked one by one on the discharge tray, the front end of the sheet to be discharged eventually abuts the bottom surface of an image reading device 60 positioned above the discharge tray 150. In this case, despite the fact that a discharge space allowing a large number of sheets to be stacked thereon is formed between the image reading device 60 and the discharge tray 150, the subsequent sheet to be discharged after the sheet abuts is jammed between the image reading device 60 and the image discharge tray 150. As a result, the subsequent sheet clogs a discharge outlet 28, thereby decreasing a stacking ability of the shacked sheets, so that the sheet sometimes falls down. As described above, in a case of a curled sheet, the conventional image forming apparatus has an issue that a sheet stacking ability decreases and that the discharge space cannot be effectively utilized.
The present invention has been accomplished in view of such circumstances, and the invention provides an image forming apparatus capable of securing a stacking ability while effectively utilizing a discharge space even when a sheet is curled.